The field F(x, y) = (−y − x(x² + y²), x − y(x² + y²)) sends every point of the plane spiralling into the origin, its only fixed point, since |F|² = r²(1 + r⁴). In polar form it separates: ṙ = −r³ and θ̇ = 1. Each particle turns at unit speed while it falls, with r(t) =